Something deleted all my calendar entries, for the second time.

I updated an entry and went to save for "this and following entries". All calendar entries were deleted: Appointments, reminders, tv schedules, you name it, probably 100 of them.

I reentered everything. Called doctor offices to get the appointment data. Each one required about 12 clicks to enter.

Today, almost 6 months later, I am updating one entry that is repeated. I hit "this and all following entries" and again, all entries are deleted.

What is happening?
What do I do to prevent it.
I believe that everything is being backed up but has the system backed up an empty calendar removing the working version?
How do I restore a version that has been backed up in my Google account?

The only information I can find about this app is in the config menu:
Calendar 6.0.0-213980623-release

Moto G

It really scares me that you suggest that other apps can delete calendar information but I am sure that this problem is entirely within the calendar and that this poor design is what is allowing me to totally clear it. I cannot imagine why some ying-tong placed an option, so easily triggered, in the normal menu and without a "Do you really want to completely clear your calendar?". Something like that should be well hidden. I speak of a saving option after I make changes.

I figured this out after p[lacing the original question. Even now, after realizing this, I see "For This and all following entries" to be the problem. It clears everything instead.

Now that I seem to figure out how it happened, how do I recover? How do I restore as you suggest in step two?